1. Field of the Invention
The present invention concerns an optical spectrum slicer for outputting discontinuous spectrum lights containing multiple spectral components each at predetermined wavelength spacing from a broad band continuous spectrum light having a wavelength region of a predetermined range, which is suitable as a light source for inspection and evaluation of optical parts, devices and systems in the dense wavelength division multiplexing optical communication or a light source for use in optical communication.
2. Statement of the Related Art
In optical communication using optical fibers as signal transmission lines, TDM (time division multiplexing) transmission has been conducted so far with an aim of higher bit-rate transmission of a great amount of information and, recently, WDM (wavelength division multiplexing) transmission has been noted for transmitting a further great amount of information along with popularization of internets.
The WDM transmission is a mode for multiplexing transmission of a plurality of optical signals of different wavelengths by way of a single optical fiber. As shown in FIG. 12, optical signals from light sources 41, . . . of different wavelength are modulated by a modulator 42, and guided by an optical multiplexer 43 into a single optical fiber 44 on the transmission side 40, while an optical signal from the optical fiber 44 is separated on every wavelength by an optical demultiplexer 46, converted into electric signals by photoreceiving devices 45 and then demodulated and taken out on the receiving side 45.
At present, transmission of signals in several tens to one hundred channels independent of each other has been put to practical use by using a single optical fiber, which can provide advantages capable of bilateral transmission, transmission of different kinds of signals such as analog signals and digital signals simultaneously, and transmission of signals at high bit-rate and of large capacity while dividing them into channels each at low bit-rate and of small capacity, by the use of light of different wavelengths.
By the way, since lights of various wavelengths transmit through optical parts, devices and systems in the WDM optical communication, it is necessary to previously detect their optical characteristic on every wavelengths as to whether each of them has intended function to all of wavelengths used.
For example, in the system as shown in FIG. 12, if the wave separation characteristic of the demultiplexer 46 depends on the wavelength, there exist wavelengths that can be separated and those that can not be separated. Further, if the photoreceiving sensitivity of each of the photoreceiving devices 47 depends on the wavelength, there exist wavelengths that can be received at high sensitivity and can not be received at high sensitivity even for the lights of an identical intensity, so that it is not preferred in view of the WDM optical communication.
Then, lights at desired wavelengths are selectively take out, by controlling the wave length of a variable wavelength laser light source or transmitting a light outputted from an light emission diode through an interference filter, and discontinuous spectrum lights having a desired wavelength spacing are entered to the optical parts, devices and systems to previously detect the characteristics of the emission light.
However, since any one of the light sources described above can output only the light of a single wavelength, when a plurality of lights of different wavelengths are intended to be multiplexed, light sources are required by the number of channel, to increase the cost.
In a case of using a wavelength variable laser and converting the light into those of different wavelength while successively adjusting the wavelength different wavelength, it may suffice to use only one light source device. However, upon entering light while varying the wavelength, it takes much time for exactly matching to an optional wavelength and a long time is necessary for evaluation of characteristics regarding all the lights, for example, in 100 channels.
Further, in the WDM transmission, it is desirable to increase the density by setting the wavelength spacing between each of transmission lights to 1 nm or less (typically about 60 to 125 GHz by frequency spacing). However, even when the laser light sources are used by the number corresponding to the number of channels, it requires high level of technique and high cost to output discontinuous spectrum lights while controlling the spacing for the wavelength of adjacent laser lights at a high accuracy of 1 nm or less.
Further, since the interference filter for use in DWDM (dense wavelength division multiplexing) transmission has a multi-layered structure of 50 to 100 layers, it is not easy to design and manufacture the filter such that discontinuous spectrum lights can be outputted at the wavelength spacing of 1 nm or less between each of adjacent lights by controlling the thickness for each of the layers even to skilled manufacturers.
Then, if discontinuous spectrum lights at a predetermined wavelength spacing for use in WDM transmission can be obtained easily, optical characteristics (wavelength dependence) of optical parts, devices and systems used for the transmission system can be examined simply.
For example, as shown in FIG. 13, when a multiplexer 43 is connected to the output of a demultiplexer 46 and discontinuous spectrum lights of known spectral characteristics are entered to the demultiplexer 46, optical characteristics of the multiplexer 43 and demultiplexer 46 can be checked easily.
In this case, when a demultiplexer 46 of known wavelength selectivity is used, the optical characteristics of the multiplexer 43 can be analyzed extremely simply. Further, when a multiplexer 43 of known wavelength selectivity is used, the optical characteristics of the demultiplexer 46 can be analyzed extremely easily.
In view of the above, it is a technical subject of the present invention to provide an optical spectrum slicer capable of outputting discontinuous spectrum lights each at a desired wavelength spacing from a broad band continuous spectrum light in an extremely simple structure and at a reduced cost, without using special light sources or filters, and further capable of matching the wavelengths of the discontinuous lights to a desired wavelength spacing.
For solving the subject, the present invention provides an optical spectrum slicer for converting a broad band continuous spectrum light having an optional wavelength region into multiple discontinuous spectrum lights each at a predetermined wavelength spacing and outputting them comprising:
a birefringent device having two polarization axes each orthogonal to an optical axis and linear polarizers disposed at the light incident end and the light emission end of the birefringent device, with the direction of polarization being inclined by about 45xc2x0 relative to each of the polarization axes, and a heat generator for variably controlling each of wavelengths while maintaining the wavelength spacing of the discontinuous spectrum lights by controlling the temperature of the birefringent device.
The term xe2x80x9cdirection of polarizationxe2x80x9d in the present specification means direction of vibration of a vibration vectors of an optical wave for light and means a direction along which the transmissibility of the linearly polarized light is maximum for the linear polarizer.
Further, the xe2x80x9cfrequencyxe2x80x9d is a function of xe2x80x9cwavelengthxe2x80x9d. Accordingly, if the term xe2x80x9cwavelengthxe2x80x9d used for describing the constitution of the present invention is replaced with the term xe2x80x9cfrequencyxe2x80x9d, this means an invention having quite technically equivalent constitution except for the expression of the term and, accordingly, such a reworded invention is also within the technical scope of the present invention.
According to the invention, when a broad band continuous spectrum light having an optional wavelength region, for example, between 800 to 3000 nm transmits the linear polarizer on the incident side, it is converted into a linearly polarized light at 45xc2x0 direction and entered to the birefringent device to form an x-polarized light and a y-polarized light each transmitting along the optical axis, in which the light intensity is identical between both of the polarized light components.
Since the refractive indexes nx and ny of the birefringent device are different with respect to the two polarization axes, a difference in the velocity is caused between the x-polarized light and the y-polarized light to form a phase difference at the emission end.
Accordingly, when the lights transmit the linear polarizer on the emission side, 45xc2x0 components of the x-polarized light and the y-polarized light are synthesized, and the identical spectral components interfere with each other, so that a comb type spectrum having spectral components at a predetermined wavelength spacing can be observed in the spectral region by wavelength scanning of the emission light by a spectral analyzer or the like.
Further, according to the experiment made by the present inventors, when the temperature for a birefringent device is elevated, the wavelength (frequency) can be shortened (made higher) while maintaining the wavelength spacing (frequency spacing) constant for each of spectral components in the comb spectrum. On the contrary, when the temperature of the birefringent device is lowered, the wavelength (frequency) can be made longer (made lower) while maintaining the wavelength spacing (frequency spacing) constant for each of spectral components in the comb spectrum.
For example, when the wavelength for each light upon WDM transmission is set to 1 nm spacing around 1550.0 nm as the center in a 1530-1600 nm band including C-band and L-band, if the wavelength of the discontinuous spectrum light outputted from the optical spectrum slicer is at 1 nm spacing around 1549.8 nm as the center, each wavelength can be made longer by 0.2 nm while keeping the 1 nm wavelength spacing thereby matching to the set wavelength by lowering the temperature for the birefringent device.
According to a second feature of the invention, since a pair of optical connectors to be connected with an optical fiber for entering a broad band continuous spectrum light and an optical fiber for emitting discontinuous spectrum lights are attached to both ends of a housing incorporating the birefringent device, with the optical axis being aligned with the optical axis of the birefringent device, they can be incorporated into the communication system by merely connecting the optical fiber to each of the optical connectors with no troublesome alignment for the optical axis.
According to a third feature of the invention, a reflection mirror is disposed for reflecting a light emitted from the birefringent device and transmitting the polarizer along the optical axis and entering the same again by way of the polarizer to the birefringent device.
In this case, an optical channel reciprocating in one birefringent device is formed and discontinuous spectral lights having more sharpened comb spectrum are outputted.
According to a fourth feature of the invention, since an optical connector for connecting an optical fiber used both for entering and emission is attached with an optical axis being aligned with the optical axis of the birefringent device, they can be incorporated into the communication system by merely connecting the optical fiber to each optical connector with no troublesome alignment for the optical axis.
According to a fifth feature of the invention, a wavelength of an optional light contained in discontinuous spectrum lights is detected and compared with a predetermined reference wavelength and the temperature of the heat generator is put to feedback control such that they are identical with each other by a wavelength controller.
According to this constitution, when the wavelength of multiple wavelength lights interfering to each other by emission from the birefringent device and transmission through the polarizer is displaced from the desired reference wavelength, feedback control is applied such that they are identical with each other to control the temperature of the birefringent device, so that discontinuous spectrum lights containing spectral components of a reference wavelength can be obtained.